"This M8 on Windows has to be a choice of HTC," said Ken Dulaney, an analyst at Gartner. "It has to involve a lot of courage of conviction."

All of the the latest M8's hardware will remain the same as the version running Android KitKat (4.4.2), but instead it will run Windows Phone 8.1.

Microsoft's newest OS features a digital voice assistant called Cortana as well as the traditional Windows Phone live tiles on the home screen. A Youtube video gives many of the details.

It is the first time a smartphone has been retrofitted to run a second OS, according to HTC. Jason Mackenzie, of HTC America, announced the new smartphone at a New York City event, saying that using the Windows Phone OS with the M8 was done "without making any compromises."

The hardware features an all-aluminum body with tapered edges, highly-rated front-facing stereo speakers and a bright LCD 5-in. display at 441 pixels per inch. A dual, rear-facing lens allows for depth perception in shots.

Darren Laybourn, corporate vice president for Windows Phone, introduced the M8 at the New York event, saying that Microsoft redesigned its Windows Phone software "so that it was worthy of the best hardware out there."

One of the biggest reasons HTC and Microsoft worked together on the M8 involves their history together. HTC has worked with Microsoft in the past, and Microsoft has shared marketing costs to promote HTC devices, analysts noted, a practice that is expected to apply to the M8.

"HTC has nothing to lose by adding a Windows Phone M8 version, especially if Microsoft is going to put their marketing money behind it," said Carolina Milanesi, chief of research at Kantar WorldPanel.

Milanesi said it is probably not right to judge Windows Phone on the second quarter's 2.5% market share, expecially because Microsoft was integrating Nokia, which later led to layoffs. During the second quarter, "carriers were sitting on the fence, there were no new products and it was a tough quarter," she said.

"It is certainly a good thing for Microsoft to be seen [with the M8 on Windows] as still running things as before the [Nokia] acquisition," she added.

HTC was originally one of the biggest suppliers of Windows Phone products, noted Jack Gold, an analyst at J. Gold Associates. "HTC needs to do something to stimulate sales, particularly in the high end, with high margin devices. By making a high-end Windows Phone device it can differentiate itself in the market, even though Windows Phone has limited market appeal. It could still generate good numbers for HTC."

So far, Microsoft and HTC haven't announced any plans to sell the M8 on Windows in Europe, where the OS does the best.

The latest M8 will compete directly with the Nokia Lumia Icon on Windows Phone, which is also sold at Verizon. The Lumia includes a 20-megapixel rear camera as its best feature.

But for Microsoft, "one added phone [like the M8 for Windows] won't make too big a difference in overall market share," predicted Patrick Moorhead, an analyst at Moor Insights & Strategy. He noted that Windows Phone has mainly suffered from arriving too long after the first iPhone and Android phones appeared.

Another persistent problem is that Windows Phone is also not attracting as much developer support to catch up, since developers prefer to keep operations simple and support the top OSes, Android and iOS.

As for HTC, it was probably influenced by Microsoft's willingness to offer the OS for free and also provide co-marketing dollars, said Rob Enderle, an analyst at Enderle Group. "HTC has to find a way to break out or they are at risk of going under and I expect they are starting to get a bit desperate," he said.

This Verizon Wireless video touts the HTC One (M8) for Windows smartphone.